1. Field of the Invention
This invention relates to a method, apparatus, and medium for magneto-optical recording capable of direct overwriting.
2. Description of the Prior Art
In magneto-optical recording, various methods for direct overwriting have been proposed to improve the data rate. They include a typical example of a light modulation method using a double-layered film, which is disclosed in JA Published Unexamined Patent Application (PUPA) 62-175948.
The contents of this application are also reported by Matsumoto et al. in "Direct Overwrite by Light Power Modulation on Magneto-Optical Double-Layered Media," Digest of 53rd Seminar, the Magnetics Society of Japan (1987), p. 87.
A recording medium used in this method has a recording layer consisting of two layers, a memory layer and a reference layer, which are exchange-coupled. Overwriting is performed by utilizing the difference in temperature dependence of the coercive forces of the two layers. FIG. 1 shows the magnetic properties, and FIGS. 2A and 2B show the overwriting process.
As shown in FIG. 1, the compositions of the two layers are adjusted so that the coercive force of the reference layer (Hr2) is smaller than that of the memory layer (Hr1) at room temperature (Tamb1), and the Curie temperature of the reference layer (Tc2) is higher than that of the memory layer (Tc1). As shown in FIGS. 2A and 2B, one of the characteristics of this method is that an initializing field, as well as a bias field for recording, is applied before data is recorded on the memory layer. The directions of the bias field and the initializing field are anti-parallel. The magnitude of the bias field Hb is set at such a small value as to maintain the magnetization of the reference layer unreversed in the L process, which will be referred to later. On the other hand, the magnitude of the initializing field Hini is set at a value larger than Hr2 but smaller than Hr1. As a result, only the magnetization of the reference layer is oriented parallel to Hini (downward in the figure). The data recorded in the memory layer is not affected by Hini.
For recording, the H process or L process is performed, depending on the bit data to be recorded. In the L process, a low-power laser beam in the form of pulses is emitted so that the temperature of the memory layer TmL becomes Tc1&lt;TmL&lt;Tc2. At this time, the magnetization of the reference layer is not reversed. Therefore, the magnetization of the memory layer is oriented in a direction determined by the exchange-coupling with the reference layer during the cooling process. The term "exchange-coupling" here means a phenomenon such that the subnetwork magnetizations of RE and TM atoms are aligned to those of similar atoms, respectively, even in different layers. Therefore, depending on the compositions of the two layers, the exchange-coupling exerted by one layer during the cooling of the other layer may result in these layers having parallel or anti-parallel directions of magnetization. FIG. 2B shows the case in which the directions of magnetization of the two layers become parallel as a result of exchange-coupling.
In the H process, a high-power laser beam in the form of pulses is emitted, with the result that the temperature of the memory layer TmH becomes Tc2&lt;TmH. Consequently, during the cooling process, the magnetization of the reference layer first coincides with the direction of the bias field (upward in the figure). That is, the direction of magnetization of the reference layer is reversed. When the temperature of the recording layer decreases, the magnetization of the memory layer is oriented in a direction determined by the exchange-coupling with the reference layer. Since the direction of magnetization of the reference layer has been reversed from that in the L process, the direction of magnetization of the memory layer is also reversed from that in the L process.
As described above, the method of JA PUPA 62-175948 needs an external field for initializing the reference layer (initializing field) before recording (by the L process or H process), in addition to an external field applied during recording (a bias field). This makes the apparatus complicated. The above method also involves the problem that data recorded in the memory layer are lost owing to the influence of the strong initializing field. Moreover, this method also involves the problem that strict requirements for the Curie temperatures and coercive forces of respective layers result in less flexibility in the selection of materials and necessitate accurate control of the compositions of materials during the preparation of media.
Some methods of eliminating the initializing field have been proposed. Among them, T. Fukami and his colleagues' "Novel direct overwriting technology for magneto-optical disks by exchange-coupled RE-TM quadrilayered films," J. Appl. Phys. 67(9), May 1, 1990 uses quadrilayered films as recording media and makes the Curie temperatures, coercive forces, and inter-layer exchange-coupling forces of respective layers different. In this method, however, the number of layers of the medium is increased to four and these layers need to satisfy certain relative requirements with respect to Curie temperatures, exchange-coupling forces, and so on. Therefore, this method not only fails to remove the restrictions on the compositions of materials, but rather increases them. In order to satisfy the requirements, highly accurate control of the composition of each layer is necessary, and hence the production cost of media becomes a problem affecting their practical use. Further, the total thickness of four layers amounts to a value of the order of 2600 Angstroms. This results in lower writing efficiency and hence requires higher laser energy.